1. Field of the Invention
The present invention relates to a code conversion circuit for optical duobinary transmission and to an optical transmitter and receiver using the same.
An optical transmission system is generally configured to receive as input an electrical signal to be transmitted directly to an optical modulator, convert this to an optical signal, and launch the thus converted optical signal to an optical fiber to transmit it to a receiving side.
In recent years, however, along with the increase in transmission capabilities, increasingly harsh demands have been placed on the characteristics of optical fibers. First, there is the demand for realization of long distance optical fiber transmission so as to enable transmission of light without repeaters. Second, there is the demand for transmission of superhigh speed signals, for example, the realization of superhigh speed optical fiber transmission exceeding 10 Gb/s.
In such superhigh speed, long distance optical fiber transmission, the quality of the transmitted signal ends up deteriorating due to the wavelength dispersion characteristic distinctive to optical fibers, which makes transmissions difficult. Therefore, optical duobinary transmission has already been proposed, as a so-called high xe2x80x9cdispersion tolerancexe2x80x9d transmission system, and studies conducted toward its commercialization.
The present invention relates to a code conversion circuit for such optical duobinary transmission.
2. Description of the Related Art
As will be explained in detail later using the drawings, a first example of a code conversion circuit called a pre-coder has already been proposed, but as will be explained later, suffers from the disadvantage that a signal of a completely different bit pattern from the desired output signal ends up being produced. To eliminate this disadvantage, a later explained second example of a pre-coder has been proposed.
This second example of a pre-coder operates normally when the delay time Td at its delay element (mentioned later) is shorter than one time slot TS, but no longer operates normally when the Td is longer than one time slot TS. In superhigh speed optical transmission, 20 Gb/s or 40 Gb/s speeds will be realized in the future. When this happens, one time slot TS will become further shorter, that is, 50 ps or 25 ps. Even if improvements in IC process technology raise the maximum operating speed and enable the circuit delay to be made smaller, the delay time due to the wirings etc. will remain unchanged. Therefore, the ratio of the delay time due to the wirings in the total delay time Td will become larger as the time slot TS becomes shorter and it will become more difficult to make the delay time Td shorter than the time slot TS. When this happens, normal operation of the pre-coder will no longer be able to be ensured.
An object of the present invention is to provide a code conversion circuit in an optical transmitter or optical receiver for superhigh speed transmission which ensures normal operation even if including a delay element with a delay time exceeding the time of one time slot of an input signal.
To attain the above object, the code conversion circuit of the present invention provides a bit distributing unit (21) for dividing a high speed input signal (IN) into N ways of low speed signals (in1, in2 . . . ), N number of code converters (22-1 to 22-N) for performing code conversion on the N ways of low speed signals, and a bit combining unit (23) for receiving as input the code-converted low speed signals (out1, out2 . . . ) output from the N number of code converters and logically adding and combining these to produce a code-converted high speed output signal OUT. As a result, it becomes possible to ease the harsh restriction of one time slot on the delay time imposed on a delay element in a pre-coder used for optical duobinary transmission.